1. Field of the Invention
The present invention generally relates to coating processes. More particularly, this invention is directed to a physical vapor deposition process and apparatus for depositing ceramic coatings containing multiple oxides and elemental carbon and/or a carbon-based gas.
2. Description of the Related Art
Certain components of the turbine, combustor and augmentor sections of a gas turbine engine are typically protected from their harsh thermal environments by a thermal barrier coating (TBC) formed of a ceramic material. Various ceramic materials have been proposed for TBC's, the most notable of which is zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3). Binary yttria-stabilized zirconia (YSZ) is widely used as a TBC material because of its high temperature capability, low thermal conductivity and erosion resistance in comparison to zirconia stabilized by other oxides. YSZ is also preferred as a result of the relative ease with which it can be deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC's employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC material has and maintains a low thermal conductivity. However, the thermal conductivity of YSZ is known to increase over time when subjected to the operating environment of a gas turbine engine. To reduce and stabilize the thermal conductivity of YSZ, ternary YSZ systems have been proposed. For example, commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a TBC of YSZ alloyed to contain certain amounts of one or more alkaline-earth metal oxides (magnesia (MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (lanthana (La2O3), ceria (CeO2), neodymia (Nd2O3), gadolinium oxide (Gd2O3) and dysprosia (Dy2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al., when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. In commonly-assigned U.S. Pat. No. 6,808,799 to Darolia et al., a TBC of YSZ is deposited to contain a third oxide, elemental carbon, and potentially carbides. The resulting TBC is characterized by lower thermal conductivity that remains more stable during the life of the TBC as a result of stable porosity that forms when the elemental carbon and carbides within the TBC oxidize to form carbon-containing gases (e.g., CO).
While the incorporation of additional oxides and carbon-containing compounds into a YSZ TBC in accordance with Rigney et al. and Darolia et al. has made possible a more stabilized TBC microstructures, it can be difficult to deposit a TBC by an evaporation process to produce a desired and uniform composition if the additional oxide has a significantly different vapor pressure (e.g., an order of magnitude) than zirconia and yttria. For example, co-evaporation of YSZ and zirconium carbide (ZrC) as a source of carbides and/or carbon is complicated by the low partial pressure of ZrC, yielding a TBC that has an unacceptable nonuniform distribution of carbides. To avoid this result, separate ingots of YSZ and ZrC may be evaporated with a single electron beam using a controlled beam jumping technique, with the dwell time on each ingot being adjusted so that the energy output achieves the energy balance required to obtain compositional control of the vapor cloud that condenses on the targeted surface to form the desired coating. Alternatively, multiple electron guns can be operated at power levels suited for the particular material being evaporated by a given gun. Yet another approach disclosed in commonly-assigned U.S. Pat. No. 6,790,486 to Movchan et al. involves regulating when vapors from one or more evaporation sources are permitted to condense on the surface being coated, such that deposition only occurs while the relative amounts of vapors within the vapor cloud are at levels corresponding to the desired coating composition.
It would be desirable if a process existed that simplified the co-evaporation of materials with different vapor pressures during the deposition of TBC's and other coatings.